Cholecystokinin octapeptide (CCK-8) occurs in relatively large amounts in some neurons of the cerebral cortex. Unlike most other mammalian neuropeptides, this compound contains a sulfate ester. We injected radiolabeled inorganic sulfate (35S-sulfate) into rat cerebral cortex and measured the formation and disappearance of radiolabeled CCK-8 using HPLC. The data allow the first calculation of the turnover rate of a non-secretory neuropeptide in brain. The turnover of CCK-8 in cortex (half-life = 16 hr) is considerably slower than that of the biogenic amines and amino-acid neurotransmitters (half lives less than 4 hr). The enzymes responsible for sulfation is phenol sulfo transferase (PST). Since ability to inhibit sulfation would make possible a non isotopic method for CCK turnover, and aid in studying post-translational modification of proteins, we also examined the ability of the best available inhibitor of PST dichloronitrophenol to block sulfation in vivo of phenols with widely differing affinity for PST and to block incorporation of SO4 into CCK-8.